The present invention relates to a sensing apparatus which detects a target substance by using evanescent light.
When light is incident on a medium of a low refractive index from a medium of a high refractive index, the light is totally reflected at an interface between the two media if its angle of incidence is set equal to or more than a critical angle. In this case, as known, evanescent light is generated to permeate the low refractive-index medium side up to about one wavelength of the light. An area where such evanescent light is generated (evanescent area hereinafter) is a very narrow area of about one wavelength from the interface, i.e., a light reflection surface. Thus, by using the evanescent light, a state of an area in the vicinity of the interface corresponding to the evanescent area can be observed.
Known examples of using evanescent light are a method and an apparatus for detecting presence of a target substance or measuring the amount of a target substance by holding the target substance labeled by a light scattering label of a fine metal particle in an evanescent area, and detecting evanescent light scattered by the light scattering label.
For example, JP 10-506190 A describes a waveguide binding assay method for scattering light by light scattering labels specifically bound to a waveguide element within a penetration depth of an evanescent wave, guiding the scattered light to the waveguide, and detecting a light scattering phenomenon in all areas of the waveguide element by a photodetector such as a CCD camera.
JP 08-500667 A describes a method for binding to binding pair elements an analyte such as an antigen immobilized to other binding pair elements such as antibodies and labeled by a light scattering label in two or more places on a single reactive surface of a cuvette made of an optically transparent material, illuminating the reactive surface with a damped wave (evanescent light wave) generated from an optical beam incident on the transparent material of the reactive surface, and detecting an elastically scattered light obtained from interaction between the damped wave and the light scattering label to detect the immobilized binding pair elements. This Publication further describes separate detection of the binding pair elements in a state being separated in two or more places.